Conventionally, there has been known, as a technology relating to display processing of a virtual machine, a technology wherein a virtual machine control program recognizes screen switching from an input device for exclusively displaying a screen image corresponding to each respective virtual machine on a common display device in accordance with a designation (see e.g. patent literature 1).
More specifically, a virtual machine control program called a hypervisor retains screen data representing a screen image to be outputted from each respective virtual machine in a screen buffer. Then, in response to receiving a request from an input device to switch from a screen image of a certain virtual machine to a screen image of another virtual machine, the virtual machine control program exclusively switches the on-screen image of the certain virtual machine to the screen image of the designated virtual machine.
In the technology disclosed in patent literature 1, however, it is impossible to combine screen images of the virtual machines, because the virtual machine control program exclusively switches the screen image of each respective virtual machine.
Further, there is also disclosed, as another technology relating to display processing of a virtual machine, a technology of dynamically controlling superimposition of images between different drawing systems, window by window, on a display such as an LCD (Liquid Crystal Display) (see e.g. patent literature 2).
Specifically, in the technology disclosed in patent literature 2, a system is provided with a processing section (1) and a processing section (2) in each of which a different OS is installed, a window (711) is drawn in a hardware layer (HW layer) (71) as a VRAM prepared in the processing section (1), and a window (712) is drawn in an HW layer as a VRAM prepared in the processing section (2). In the case where the priority of the HW layer (71) is lower than the priority of the HW layer (72), the window (711) drawn in the HW layer (71) is detected as a specific window, and a transparent window (725) having the same position and the same size as those of the specific window is drawn in the HW layer (72). Then, the HW layer (71) and the HW layer (72) are combined by an LCD controller, and a combined image in which the window (711) and the window (712) are superimposed one over the other is displayed on a display screen (41). Thereby, the window (71) drawn in the HW layer (71) is displayed in front of the window (712) drawn in the HW layer (72) through the transparent window (725) drawn in the HW layer (72).
In the technology disclosed in patent literature 2, the LCD controller is operable to combine the HW layer (71) in which the window (711) is drawn, and the HW layer (72) in which the transparent window (712) and the window (712) are drawn. It is, however, uncertain whether the above technology is feasible, because there is no specific description about a manner as to how the LCD controller combines the layers.
Further, the technology disclosed in patent literature 2 has a problem that the window (711) drawn in the HW layer (71) is displayed in front of the window (712) drawn in the HW layer (72), regardless of the fact that the priority of the HW layer (72) is higher than the priority of the HW layer (71).
In this section, in patent literature 2, let us consider a case that the transparent window (725) is a window having a specific key color (e.g. black), and the LCD controller combines the HW layer (71) and the HW layer (722) by drawing the window (711) that has been drawn in the HW layer (71) in a region where the key color is painted in the HW layer (72). In this case, if another window which is semi-transparent is superimposed on the HW layer (72), the color of the transparent window (725) changes from the key color to another color. As a result, it is impossible to draw the window (711) that has been drawn in the HW layer (71) in the transparent window (725).
There is a case that secure virtual machines and non-secure virtual machines are provided in a virtual machine system configured of a plurality of virtual machines. In such a case, it is required to inhibit a non-secure virtual machine from accessing screen data to be drawn by a secure virtual machine.
However, patent literatures 1 and 2 are silent about an arrangement taking into account co-existence of non-secure virtual machines and secure virtual machines. Thus, there is a problem that a non-secure virtual machine may access screen data representing a screen layer to be drawn by a secure virtual machine.
If a virtual machine system provided with secure virtual machines and non-secure virtual machines is configured in such a manner that a secure virtual computer combines screen layers generated in both of the secure virtual machine and a non-secure virtual machine, it is possible to protect the screen layer generated in the secure virtual machine from the non-secure virtual machine.
There is also another problem that frequent generation or erasing of a screen layer in a non-secure virtual machine results in frequent data transmission/receiving between a secure virtual machine and the non-secure virtual machine, which may degrade the performance of the entirety of the system.